My first thought back then with my C64 and BASIC: "I'll make it so when someone types LIST they won't be able to see the code, by writing more BASIC code that prevents it".
REM<SHIFT-L> - it's weird the things that stick in your head after all this time. Likewise SYS 64738, 64760, 64767 and a few others. One to really mess with people was:
POKE 53280, 0 # set screen to black
POKE 53281, 0 # set font/foreground black
SYS 64767 # fast restart which doesn't unset colours. At that point unless they know what's happened they cant RUN-STOP/RESTORE to get out of anything.
I do miss the 64 (and the Amiga which followed). The variety of machines back then was really refreshing compared to now.
